Optical communication networks which are becoming faster in speed and larger in capacity have resulted in an increased need for apparatuses for performing optical signal processing typified by processing of optical signals in the form of wavelength division multiplexing (WDM). For example, such a function is demanded with which network route switching is performed at a node on a multiplexed optical signal which has not undergone optical to electrical conversion and hence which is still an optical signal. Thereby, transparent networks are under development.
Meanwhile, waveguide-type optical circuits (PLC: Planar Lightwave Circuit) have been researched and developed from the view points of size reduction and integration of optical signal processing apparatuses. In a PLC, for example, cores made of silica glass are formed on a silicon substrate, and various functions are integrated on a single PLC chip. Thereby, an optical functional device with low loss and high reliability has been achieved. Moreover, there exist composite optical signal processing devices (apparatuses) in each of which multiple PLC chips and other optical functional devices are combined.
For example, Patent Document 1 discloses an optical signal processing apparatus in which a waveguide-type optical circuit (PLC) including an arrayed-waveguide grating (AWG) and the like is combined with a spatial light modulator such as a liquid crystal device. Specifically, studies have been carried out on wavelength blockers each formed of a PLC and a collimating lens which are symmetrically arranged about a liquid crystal device, wavelength equalizers, dispersion compensators, and the like. Such optical signal processing apparatuses perform optical signal processing on multiple optical signals with different wavelengths independently of each other on a wavelength basis.
In an optical signal processing apparatus using a PLC, it is necessary to optically couple various bulk type optical devices to a PLC, or PLCs to each other. In general, for establishment of optical connection of a bulk type optical device or the like in free space optics, it is necessary to convert optical signals propagating in atmosphere into collimated beams (parallel beams) and then to input and output the collimated optical signals for the purpose of preventing loss due to widened optical signals. Moreover, the collimated beams need to have an appropriate predetermined beam diameter in accordance with optical characteristics of the bulk type optical device. For this end, for example, a collimating lens or the like is used.
FIG. 10 is a drawing showing a part of a structure of a conventional optical signal processing apparatus using a PLC. In the structure shown in FIG. 10, a cylindrical lens is used to collimate output light from a facet of the PLC. The optical signal processing apparatus in FIG. 10 has a structure in which optical devices are sequentially fixed in alignment with an optical base plate 106 taken as a reference. A PLC 101 and a cylindrical lens 102 are fixed to optical alignment parts 103a, 103b, and 104, for example, by caulking into metal plates, such as stainless steel plates, or metal frames or by using a low-melting glass, a solder, an adhesive, or the like. Here, the optical alignment part 103a and the optical alignment part 103b may be a single integrated part. In FIG. 10, a side face portion is not shown so that the cylindrical lens 102 can be seen. A part may be provided to the side face portion. The above-described PLC 101 and cylindrical lens 102 are respectively fixed to metal bases 105a and 105b. The bases 105a and 105b can slide on the optical base plate 106, so that their positions on a plane can be adjusted. The bases 105a and 105b are provided with joint portions which allow also heights and facing directions of the bases 105a and 105b to be adjusted to some extent. An optical signal 100 inputted to the PLC passes through the cylindrical lens 102, and is outputted as collimated beams.
The above-descried structure of the conventional technique requires the following assembly processes of the optical signal processing apparatus. Specifically, the assembly processes include a process of fixing the PLC 101, the lens 102, and the like to the optical alignment parts 103a, 103b, and 104 with an adhesive or the like; a process of fixing these optical alignment parts to the bases 105a and 105b; and further a process of aligning and fixing these bases to the optical base plate 106. In some cases, it is also necessary to fix the bases to the optical base plate by YAG laser welding or the like. Operation for each of these processes is complicated and time consuming.
In the above-described structure of the free space optics, ends from which optical signals output are exposed to a space. Hence, it is necessary to form anti-reflection coatings on output facets of the lens 102 and the PLC 101. Condensation or dust adhesion may occur between the output facet of the PLC 101 and the lens. Moreover, it is necessary to make a mechanical design in accordance with weights of all mechanical parts and in consideration of vibration. Moreover, when the output facet of the PLC is polished with an angle in order to prevent return light from occurring at the facet, there has been a problem that the following coupling loss occurs.
FIG. 11 is a diagram of behavior of output beams at and around a facet of a PLC of a conventional technique when viewed from a side face of a substrate. The facet of the PLC 101 is polished more toward an upper surface of the PLC substrate so that the facet can be angled slightly. Suppose a case where optical signals output from this angled facet of the PLC. If an air gap exists before the lens, a vertically asymmetric distortion which cannot be corrected even by use of an ordinary asperic lens is caused in a beam shape. FIG. 11 shows this distortion in the beam shape using lines with arrows representing optical paths. Specifically, FIG. 11 shows that an optical signal through an optical path passing a center of the lens travels in a horizontal direction, whereas optical signals through upper and lower optical paths passing peripheral portions of the lens travel upwardly from the horizontal direction. This results in a reduction in coupling efficiency between PLC-guided light and Gaussian beams propagating in atmosphere, which causes an excessive coupling loss.
As has been described above, the structure of the conventional technique, in which optical signals outputting from the facet of the PLC are optically coupled to a different bulk type optical device or the like, has a problem of being so complicated that an assembly process thereof is time consuming and alignment therefor is laborious. There has been demand of an optical circuit which allows an optical signal processing apparatus to be easily produced and assembled and which has a simpler structure. Moreover, the structure in which the output facet of the PLC is polished with an angle in order to prevent return light from occurring at the facet has a problem of an increase in coupling loss in free space optics.
The present invention has been made in view of such problems. An object of the present invention is to achieve an optical signal processing apparatus producible in a simpler assembly process by optically coupling optical signals outputting from a facet of a PLC to a different bulk type optical device or the like, by use of an optical circuit with a simpler structure. Moreover, another object of the present invention is to prevent reduction, due to the angled facet, in coupling efficiency between PLC-guided light and Gaussian beams propagating in atmosphere and to reduce coupling loss in free space optics.
Patent Document 1: Japanese Patent Laid-Open No. 2002-250828 (pages 16, 19, FIGS. 20, 27 29D, and the like).